A typical molten metal facility includes a furnace with a pump for moving molten metal. During the processing of molten metals, such as aluminum and zinc, the molten metal is normally continuously circulated through the furnace by a centrifugal circulation pump to equalize the temperature of the molten bath. These pumps contain a rotating impeller that draws in and accelerates the molten metal creating a laminar-type flow within the furnace.
To transfer the molten metal out of the furnace, typically for casting the metal, a separate centrifugal transfer pump is used to elevate the metal up through a discharge conduit that runs up and out of the furnace. As shown in FIG. 1, a typical prior art transfer pump includes a base 5, two to three support posts 6 (only one shown), a shaft-mounted impeller 7 located within a pumping chamber or volute 5a in the base 5, a motor 8 and motor mount 9 which turn the impeller, bearings 10 that support the rotating impeller (and shaft), and a riser tube or conduit 11 located at the outlet of the base. The riser 11 is provided to allow the metal to lift upward over the sill edge of the furnace in order to transfer some of the molten metal 12 out of furnace into ladles or molds.
A well-known problem with previous transfer pumps, however, is that the relatively narrow riser tube 11 becomes clogged as small droplets of the molten metal accumulate in the riser each time the pump stops transferring and the metal stops flowing through the riser. Initially, the metal accumulates in the porosity of the riser tube material (typically graphite or ceramic) and then continues to build upon the hardened metal/dross until a clog 13 occurs. As a result of this problem, furnace operators must frequently replace the transfer pump's riser tube as they are too narrow to effectively clean. This replacement typically requires the furnace to be shut down for an extended period to remove the clogged riser tube.
Several treatments have been used to alleviate this riser-clogging in transfer pumps. Including impregnating, coating, and inert gas pressurization of the riser to reduce the build-up within the tube. Another method pump manufacturers employ is to simply increase the diameter of the riser to delay the blockage. These treatments have varying degrees of success, but still only delay the inevitable clogging of the riser.
Another common operation in a molten metal facility is to add scrap metal, typically metal working remnants or chips, to the molten bath within a furnace. The heat of the bath melts the chips. Currently, the added chips are simply allowed to fall into the bath or may be mixed into the molten metal by a circulation pump. The current process(es), however, is not effective to fully immerse the solid chips into the molten bath resulting in a longer melt time.
In view of the current inefficient use of molten metal transfer pumps, there is a need for a molten metal pump that overcomes all of the above-indicated drawbacks of prior transfer pumps.